With a £20,000 budget we're assuming that you're looking for a used car and not a new one, in which case a good option if you like the Mercedes-Benz A-Class is the company's Mercedes GLA, which is a small A-Class based SUV. If you'd rather not go for an SUV, then a car with similarly Germanic quality, that's inexpensive to run but spacious enough to give your parents better space and access to the rear is the BMW 2-Series Active Tourer, which will be available second hand well within your budget and is a very enjoyable car to drive, as well as a practical one.